In the process of making polycarbonate, reactions which lead to a phenyl end group result in a decrease in reaction rate and chain growth. If such phenyl capping reactions happen with high frequency, the average length of polymer molecules in a composition will be short compared to a composition where capping reactions happened with lower frequency.
Because of this, the properties of a polymeric material are related to the proportion of polymers which have been terminated. The "end-cap level" is a quantitative measure of this proportion, expressed as a percentage. The end-cap level is arrived at by determining the number of chains which are terminated with a reactive hydroxyl group (uncapped) and then taking the remainder of the chain ends as being capped. Such a determination can be made using spectroscopic measurements. The level of endcapping (E/C%) is then given by the formula: EQU E/C%=(capped chain ends/total chain ends).times.100
One method for manufacture of polycarbonates is based upon the melt polycondensation of aromatic dihydroxy compounds such as bisphenol A (4,4'-dihydroxydiphenyl-2,2-propane, BPA) with carbonic acid diesters such as diphenylcarbonate (DPC) in the presence of an alkaline catalyst. In conventional processes of this type, the reactants are sequentially subjected to conditions which form a melted mixture or reactants, form low molecular weight prepolymers, and form the final product from the low molecular weight prepolymers. This stepwise approach to processing facilitates the production of a consistent product with well defined characteristics. Thus, polycarbonates may be prepared in a multistage reactor system such as that shown in FIG. 1, where the reactants BPA and DPC are first combined with a catalyst such as tetraalkylammonium hydroxide and a basic alkali metal catalyst in a mixing stage (MD). In the mixing stage, the reactants and catalyst are mixed together and heated to form a melt. This melt is then transported into a first reactor (1R), where the transesterification reaction of BPA and DPC begins to start the formation of prepolymers. The product produced in this first reactor is principally small condensation products (oligomers) and unreacted starting materials. In the second reactor (2R), the size of the oligomers formed is larger, as a result of continued reaction of the initially formed oligomers. After the second reactor, the melt is transported to a first polymerizer (1P), such as a double screw stirring polymerizer. In this polymerizer, the prepolymers are processed at a first polymerization temperature, for example 290.degree. C. for a period of time such as 5 minutes. This processing results in the formation of a polycarbonate product which contains residual catalyst. Because this catalyst can compromise the properties of the final product if allowed to remain, a quencher is added which neutralizes the catalyst, and the quenched product is fed to a second polymerizer in which final processing occurs by evaporation of unreacted residual monomers to produce a low viscosity (e.g., IV.apprxeq.0.35 g/dl; Mn.apprxeq.8500 g/mol) polycarbonate product.
To increase the flexibility of a polycarbonate manufacturing facility such that either low or medium viscosity (IV.apprxeq.0.42 g/dl; Mn.apprxeq.10,800 g/mol) products can be produced, a processing scheme which utilizes two alternative second polymerizers as shown in FIG. 2 can be used. In this case, the product stream from second polymerizer 2aP is substantially the same as the product stream from the series of steps shown in FIG. 1. The product stream fed to the other second polymerizer 2bP is not quenched, however, such that the processing in the polymerizer results in a further increase in the size of the products and a medium viscosity product.
The products produced in each of these reactions generally have end-cap levels of about 75-85%. Such levels are suitable for many applications. However, certain applications, such as the manufacture of optical disks, require low viscosity polycarbonates with higher and consistent end-cap level greater than 90% in order to achieve desired characteristics including antistatic properties. To date, there is no efficient method for manufacturing polycarbonates while providing consistent and controlled high end-cap levels.
It is a goal of the present invention to provide a method for manufacturing low viscosity-high end-cap level polycarbonates via a melt condensation procedure.
It is a further goal of the present invention to provide a branched processing procedure in which either low viscosity-high end-cap level polycarbonates or medium viscosity, normal end-cap products are made via a melt condensation procedure.